Ionic fluoropolymers (e.g., fluorinated ionomers or fluorinated polyelectrolytes) are polymers that have covalently bound ionic groups such as, for example, —SO3−. In solid form, the ionic groups are typically present as salts with cationic species (e.g., with H+, Na+ or Zn2+), while in aqueous solutions the ionic groups exist as the free —SO3− form. Ionic fluoropolymers are used to manufacture membranes that are used in fuel cells. Such membranes are typically formed by casting a dispersion of ionic fluoropolymer in an aqueous medium on a substrate and subsequent removal of the aqueous medium.
Such membranes typically should have low levels of anionic impurities in order for the fuel cell to function properly, while low levels of cationic impurities in the membrane are typically associated with increased durability, performance, and membrane quality.
Known processes for reducing anionic impurities such as, for example, rinsing or extracting rely on purification of coagulated ionic fluoropolymer. After reducing anionic impurities, the coagulated ionic fluoropolymer is generally re-dispersed in an aqueous medium in an expensive process that typically involves harsh conditions and highly specialized equipment.
Subsequently, cationic impurities are removed by an exchange process, and the purified dispersion is then cast onto a substrate to form a membrane that is incorporated into a fuel cell.